Apparatus and method for operating a vapor generator at subcritical and supercritical pressures



March 29, 1966 w. HROEDTER 3,242,911

APPARATUS MET FOR OPERATING A VAPOR GENERATOR AT SUBCRITICAL AND SUPERCRITICAL PRESSURES Filed Dec. 23, 1.963

SUPER- HEATING W C Mb 266 VALVE 20 I96 30 FIG.3

PRE- HEATING l2 ZONE L I INVENTOR.

BY @AMQZZ/ AGENT United States Patent 3,242,911 APPARATUS AND METHOD FOR OPERATING A VAPOR GENERATOR AT SUBCRETICAL AND SUPERCRITHCAL PRESSURES Willburt W. Schroedter, West Hartford, Conn., assignor to Combustion Engineering, inc, Windsor, Conn, a corporation of Delaware Filed Dec. 23, 1963, Ser. No. 332,684 6 Claims. (Cl. 122-406) The invention relates generally to vapor generators operating at normal load in the supercritical pressure range, and is particularly concerned with a method and apparatus of operating such generators in the subcritical pressure range at lower loads than maximum load.

The critical temperature of a gas is generally defined as the temperature at which the gas may just be liquified at its critical pressure. For water or water vapor this pressure is 3206.3 p.s.i.a. (pounds per square inch absolute) at the critical temperature of 705.34 P. When heating water at a pressure below the critical pressure it passes from the liquid state to the vapor state at a constant temperature the so-called saturation temperature. A certain amount of heat is required to change the water from the liquid state to the vapor state. This heat quantity is called heat of evaporation. It decreases with an increase in pressure and becomes zero at the critical pressure. When heating water at the critical pressure or at a supercritical pressure with zero heat of evaporation a single phase condition therefore prevails, with the temperature constantly increasing while heat is added. When heating water at subcritical pressure to a two-phase condition prevails, with the temperature rising to the evaporation point, remaining constant until all water is evaporated to steam, and then rising again in the process of superheating the steam.

A number of important advantages are being derived from operating a supercritical vapor generator at reduced pressure during low load operation.

These advantages include a considerable reduction in feed pump power, and a marked reduction in superheater tube stress. The latter advantage may be expressed in one of two ways for a given superheatermaterial selection: (1) as an extension of the load range over which superheater outlet steam temperature may be maintained at its rated full load value or, (2) as an increase in the expected life of a superheater tube resulting from the lowered stress level.

It is accordingly an object of the invention to increase the over-all economy of operating a supercritical vapor generator at lower loads by reducing the operating pressure at such loads to a pressure within the subcritical pressure range.

A further object of the invention is to accomplish the above with a minimum of capital investment for additional apparatus such as valves, piping, separators, etc.

Other objects and advantages of the invention will become apparent from the following description of an illustartive embodiment thereof when taken in conjunction with the accompanying drawings wherein:

FIG. 1 is a diagrammatic representation of a vapor generator organized for both supercritical and subcritical pressure operation;

FIG. 2 depicts a vapor generating plant including a modified once-through vapor generator operating at supercritical pressure, which generator when at subcritical pressure will operate as a controlled circulation vapor generator;

FIG. 3 depicts a vapor generating plant including a once-through vapor generator operating at super-critical 3,242,911 Patented Mar. 29, 1966 pressure, which generator when at subcritical pressure will operate as a natural circulation vapor generator;

FIG. 4 is a diagrammatic representation of a threeway valve when in the position for supercritical operation; and

FIG. 5 when in the position for subcritical operation.

Referring now to the drawing wherein like reference 7 characters are used throughout to designate like elements, the illustrative embodiment of the invention depicted in FIG. 1 includes a vapor generator designated generally as 10, and comprising in the flow path of the working medium a preheating zone 12, a conduit 13, a separating and mixing zone 14, a conduit 15-, a recirculating pump 16, a radiant heating zone 18, a conduit 19, a three-way valve 20 and a superheating zone 21. Fuel and air for burning is discharged into the vapor generator by way of burner 22 and air duct 24, respectively.

When operating the vapor generator at supercritical pressure and with a modified once-through flow pattern a through-flow quantity of the working fluid passes through the preheating zone 12 into the separating and mixing zone 14. In this zone a relatively small recirculated portion of the working fluid having been returned from the outlet of the radiant heating zone 18 by way of conduit 26 is mixed with the through-flow quantity to form a total quantity. This total quantity then passes through the radiant heating zone 18, with the through-flow quantity continuing through conduit 19 and three-way valve 20 into superheating zone 21, while the above-mentioned recirculated portion by means of pump 16 and conduit 26 is returned to the separating and mixing zone 14. When operating at supercritical pressure as above described the three-way valve 20 occupies the position shown in FIG. 4 with the through-flow quantity passing therethrough in the direction indicated by the arrow.

When operating the vapor generator at subcritical pres sure such as at low loads, and with a controlled circulation flow pattern the through-flow quantity of the working fluid passes through the preheating zone 12 into the separating and mixing zone 14, wherein it is mixed with a relatively large recirculated portion returned from the outlet of the radiant heating zone 18 by way of conduit 26. The total quantity then passes through the radiant heating zone 18 for the generation of steam at constant temperature. The resultant mixture of steam and water is then returned to the separating and mixing zone 14 wherein apparatus is provided for separating the steam from the water. The water again then passes through the radiant heating zone 18 and the steam flows to the superheater zone 21 via conduit 28, three-way valve 20 and the downstream portion of conduit 19. When operating at subcritical pressure as hereinabove described valve 20 occupies the position shown in FIG. 5 with the through-flow quantity passing therethrough in the direction indicated by the arrow.

While FIG. 1 shows the basic idea of the invention in a simplified and diagrammatic form, FIGS. 2 and 3 illustrate two applications thereof in a more detailed and specified manner.

Thus, in the steam power plant shown in FIG. 2 feedwater is taken from the condenser 30 and is delivered under pressure into economizer 12a by way of conduit 32 and feed pump 33. Economizer 12a corresponds to the preheating zone 12 of FIG. 1. The preheated water is then delivered to drum 14a wherein the feedwater is mixed with a recirculated portion of the working fluid returned from the furnace tubes 18a. Drum 14a and tubes 18a correspond to the separating and mixing zone 14 and radiant heating zone 18 of FIG. 1, respectively.

When operating at supercritical .pressure the throughflow quantity of the working fluid is delivered directly from furnace tubes 18a to superheater 21a by way of conduit 19a and valve 34. Conduit 19a and superheater 21a correspond to conduit 19 and superheater 21 of FIG. 1, respectively. A second valve 35 provided in conduit 28a is closed during supercritical pressure operation. In the embodiment of FIG. 2 valve 34 performs one of the two functions of the three-way valve 20 of FIG. 1, namely that of controlling the flow from radiant heating zone 18 to superheating zone 21, and valve 35 performs the other function, namely that of controlling the flow from vessel 14a to superheater 21a. After having absorbed the amount of heat required for raising the steam temperature to a desired temperature level the steam at supercritical pressure is conducted to one or more steam turbine generators 36, with the steam subsequently being condensed in condenser 30, thereby returning to the beginning of the cycle.

The steam power plant illustrated by FIG. 2 and as hereinabove described as operating in the supercritical pressure range includes a steam generator of the modified once-through flow type also generally known as a combined circulation steam generator. The feature which distinguishes the combined circulation steam generator from the throughflow steam generator is the provision for recycling of a small fraction of the working fluid from the outlet of the radiant zone 14 to the inlet thereof (see conduit 26 of FIG. 1 or 26a of FIG. 2). This recycling of the working fluid serves the purpose of controllably maintaining a minimum velocity of the working medium passing through the furnace tubes for the purpose of satisfactorily protecting these tubes, with this velocity being independent of the through-flow quantity dictated by steam generator load.

When operating the combined circulation steam generator of FIG. 2 at subcritical pressure in accordance with the invention, the flow pattern of the working fluid is changed to the flow pattern of a controlled steam generator which requires a steam and water drum for separation of the steam from the water. This is in contrast to supercritical pressure operation when a steam and water drum is not required. However, in a combined circulation steam generator operating at supercritical pressure it is desirable to provide a mixing vessel 14 (FIG, 1), 14a (FIG. 2), for mixing the through-flow quantity with that portion of the working fluid that is returned from the outlet of the radiant heating zone 18, 18a. In accordance with the invention the mixing vessel 1411 now also serves the purpose of the steam and water drum which is required when operating the steam generator at a subcritical pressure. The flow path of the working fluid when operating in the subcritical pressure range is therefore as follows:

Feedwater flows from the condenser 30 via conduit 32, feed pump 33, economizer 12a and conduit 13a into mixing vessel or steam and water drum 14a, wherein the feedwater mixes with water returned from the outlet of furnace tubes 18a. This combined water quantity then flows to the furnace tubes 18a by way of conduit 15a and recirculating pump 16a. While passing through furnace tubes 18a a portion of the water is evaporated to steam with the water and steam mixture being discharged into the steam and water drum 14a by way of conduit 26a, and with valve 34 in conduit 19a being closed. Conventional equipment is provided in steam and water drum 14a for the separation of the steam from the Water. An amount of steam equal to the through-flow quantity then flows from vessel 14a to the superheater 21a by way of conduit 28a and valve 35 which valve is now in the open position. Having been heated in superheater 21a to the desired steam temperature the steam is then conducted to steam turbine generator 36 and is subsequently condensed in condenser 30 thereby completing the steam cycle.

In FIG. 3 the invention is applied to a straight throughflow steam generator when operating at top load in the supercritical pressure range. However when operating in the subcritical pressure range at low loads this steam generator is operating in the manner of a natural circulation boiler.

Considering first operation at full load and at supercritical pressure the feedwater is taken from condenser 30, forced through economizer 12b into furnace tubes 18b by feed pump 33. These above-named elements 12b and 18b of FIG. 3 correspond to preheating zone 12 and radiant heating zone 18 of FIG. 1, respectively. From furnace tubes 18b the heated working fluid flows directly to superheater 21b by way of conduit 1% and valve 34. Under these operating conditions valve 40 in conduit 26b and valve 35 in conduit 28b are closed. From superheater 21b which corresponds to superheating zone 21 of FIG. 1, the steam is delivered to turbine generator 36 and condenser 30 thereby completing the steam cycle.

When operating the steam generator illustrated in FIG. 3 at lower loads and in the subcritical pressure range, the steam and water circulates and flows in the manner of a natural circulation boiler. Accordingly, the feedwater passes from economizer 12b into drum 14b, where it is mixed with a relatively large quantity of recirculated water which is returned fromthe outlet of the furnace tubes 18b. This mixture in conduit 15b possesses a greater density than the density of the mixture of steam and water rising in the furnace tubes 18b, and accordingly flows to the inlet of these tubes by way of duct 15b. A portion of the water is evaporated to steam while passing through tubes 18b. The water and steam mixture thus produced is returned to drum 14b by way of conduit 26b with valve 40 being open and valve 34 in conduit 1% being closed. Conventional apparatus provided in drum 14b serves to separate the steam from the water. The steam then passes through conduit 28b, valve 35, through superheater 21b and to turbine 36. The water separated from the steam returns to the inlet of tubes 18b by gravity.

While I have illustrated and described two preferred embodiments of my invention it is to be understood that such are merely illustrative and not restrictive and that variations and modifications may be made therein without departing from the spirit and scope of the invention. I therefore do not wish to be limited to the precise details set forth but desire to avail myself of such changes as fall within the purview .of my invention.

I claim:

1. A once-through forced flow type vapor generator organized for operation alternatively at supercritical pressure and at subcritical pressure, the combination comprising:

a working fluid preheating means;

a fluid separating and mixing vessel;

a working fluid radiant heating means;

a working fluid superheating means;

means for passing a through-flow quantity of the working fluid through said preheating means, said fluid separating and mixing vessel and said radiant heating means in the order named;

means for passing an additional quantity of said working fluid through said separating and mixing vessel and said radiant heating means and for returning said additional quantity to said separating and mixing vessel;

first conduit means for passing only a quantity of said Working fluid that is equal to said through flow quantity from said radiant heating means to said superheating means;

second conduit means for passing only a quantity that is equal to said through flow quantity from said separating and mixing vessel to said superheating means;

valve means in said first conduit means and in said second conduit means;

a first position of said valve means whereby when operating at supercritical pressure said valve means ing fluid through said preheating means, said fluid separating and mixing vessel and said radiant heating means in the order named;

means for passing an additional quantity of said work- 6 a first conduit for passing only a quantity of said Working fluid that is equal to said through-flow quantity from said furnace heating surface to said superheater;

are opened for passage of said through-flow quantity 5 a second conduit for passing only a quantity that is through said first conduit means, and are closed for equal to said through-flow quantity from said seppreventing flow through said second conduit means arating and mixing vessel to said superheater; and said fluid separating and mixing vessel can funcvalve means operatively connected to said first conduit tion as a mixing device; and to said second conduit;

and a second position of said valve means, whereby 10 a fi st position of said valve means, whereby when opwhen operating at subcritical pressure aid valve erating at supercritical pressure said valve means are means are closed for preventing flow through said opened for passage of said through-flow quantity first conduit means, and are opened for passage of through said first conduit, and said valve means are said through-flow quantity through said second con- Closed for Preventing flow through Said Second duit means and said separating and mixing vessel conduit and said separating and mix ng vessel funcfunctions as a separating device. t ns as a miXing device;

2. A once-through forced flow type vapor generator and a Second Position of Said Valve means f y organized for operation alternatively at supercritical pres- When operating at subcritical pressure and with consure and at subcritical pressure, the combination comtrolled ir i W Said first valve means are prising: closed for preventing flow through said first cona working fluid preheating means; d-uit, and said valve means are opened for passage a fluid separating and mixing vessel; of said through-low quantity through said second a working fluid radiant heating means; conduit and said separating and mixing vessel funca working fluid superheating means; tions additionally as a separating device.

means for passing a through-flow quantity of the work- A modified Ough flow yp Vapor generator organized for operation alternatively at supercritical pressure and at subcritical pressure as a controlled circulation vapor generator, the combination comprising:

an economizer;

ing fluid through said separating and mixing vessel a fluid sepal'ating'and miXing Vessel;

and said radiant heating means and for returning furnace tubular heating Surface;

said additional quantity to said separating and mixa superheater;

ing vessel; a feed pump for passing a through-flow quantity of the first conduit means for passing only a quantity of said Working fluid through Said economilfil, Said fluid working fluid th t i equal to id throu h fl separating and mixing vessel and said furnace heating quantity from said radiant heating means to said Surface/in the Order named;

superheating means; recirculating means including a recirculating pump for second conduit means for passing only a quantity that Passing an additional q y of Said Working fluid is equal to said through-flow quantity from said through Said Separating mixing Vessel and Said separating and mixing vessel to said superheating fflmacil heating Surface and for returning Said addimeans; tional quantity to said separating and mixing vessel; first valve means in said first conduit means; a first Conduit for Passing only a q y of said Worksecond valve means in said second conduit means; ing fluid that is equal Said through-flow q y whereby when operating at supercritical pressure said from Said furnace heating surface to Said p first valve means are opened for passage of said mater;

through-110W quantity through Said first conduit 40 a second conduit for passing only a quantity that is means, and said second valve means are closed for equal to Said tl'lmugh'flow quantity from said ppreventing fl th h i Second conduit means arating and mixing vessel to said superheater;

and id fluid separating and mixing vessel can func a first valve in said first conduit;

m as a i i d i a second valve in said second conduit; and when operating at subcritical pressure and natural whereby when operating at Supfircritical Pressure Said circulation flow said first valve means are closed for first Yalve is opened for Passage of Said thmugibflow preventing flo through said first conduit means quantity through said first conduit, and said second and aid second valve means are opened for valve is closed for preventing flow through said secsage of said through-flow quantity through said ,c 0nd (fondult i swarming mixing Vessel second conduit means and said separating and mixfuncnons as a nilxmg devlcei i vessel fu ti additionally as a Separating and when operatingat subcritical pressure and with device controlled circulation flow said first valve is closed 3. A modified once-through flow type vapor generator preventing flow.thmugh Said first conduit: and

organized for operation alternatively at supercritical pres- Sald second Valve opened Passage of said sure and at subcritical pressure with a controlled circul-athrough-flow quantity through Sald Second conduit and said separating and mixing vessel functions additionally as a separating device.

5. The method of operating a modified once-through forced flow type vapor generator alternatively at supercritical pressure and at subcritical pressure as a controlled circulation vapor generator, said generator having a preheating zone, a separating and mixing zone, a radiant heat- 1ng zone and a superheating zone arranged in the order named for flow of the working medium therethrough, the steps comprising:

(1) preheating a through-flow quantity of the working medium while flowing it through said preheating zone;

(2) conducting said preheated through-flow quantity to the separating and mixing zone;

(3) forming a total quantity by mixing in said separating and mixing zone said through-flow quantity with a recirculated quantity of the Working medium that is returned from the outlet of said radiant heating zone;

(4) when operating at supercritical pressure and with a modified once-through flow pattern, (a) heating said total mixed quantity by passing it through said radiant heating zone, and thereafter, (b) superheating an amount of said total quantity that is equal to said through-flow quantity by passing it through said superheating zone, and, (c) returning the remaining quantity to said separating and mixing zone as aforesaid;

(5) or, When operating at subcritical pressure and with a controlled circulation flow pattern, (a) separating in said separating and mixing zone from said total quantity an amount of vapor that is equal to said through-flow quantity and, (b) heating said vapor quantity by passing it through said superheating zone, while (c) heating the remaining quantity by passing it through said radiant heating zone and thereafter, (d) returning the remaining quantity to said separating and mixing zone as aforesaid.

6. The method of operating a forced through flow steam generator alternatively at supercritical pressure and at subcritical pressure as a natural circulation steam generator, said generator having an economizer, a separating and mixing vessel, furnace tubular heating surface and a superheater for flow of the Working medium therethrough, the steps comprising:

(1) preheating a through-flow quantity of the working medium while flowing it through said economizer;

(2) conducting said preheated through-flow quantity to the separating and mixing vessel;

(3) forming a total quantity by mixing in said separating and mixing vessel said through-flow quantity With a quantity of the Working medium that is received from the outlet of said furnace tubular heating surface;

(4) when operating at supercritical pressure and with a forced through flow pattern, (a) heating said total quantity by passing it through said furnace tubular heating surface, and thereafter, (b) heating an amount of said total quantity that is equal to said through-flow quantity by passing it through said superheater, with (c) the remainder being returned to and received by said separating and mixing vessel as aforesaid;

(5) or, when operating at su'bcritical pressure and with a natural circulation flow pattern, (a) separating in said separating and mixing vessel from said total quantity an amount of steam that is equal to said through-flow quantity, and (b) heating said steam by passing it through said superheater while (c) heating the remaining quantity by passing it through said furnace tubular heating surface and (d) returning it to said separating and mixing zone as aforesaid.

References Cited by the Examiner UNITED STATES PATENTS 2,989,038 6/1961 Schwarz l22--406 3,038,453 6/1962 Armacost 122406 3,046,956 7/1962 Kuljian et a1. l2245l 3,135,243 6/1964 Schroedter 122-406 KENNETH W. SPRAGUE, Primary Examiner. 

1. A ONCE-THROUGH FORCED FLOW TYPE VAPOR GENERATOR ORGANIZED FOR OPERATION ALTERNATIVELY AT SUPERCRITICAL PRESSURE AND AT SUBCRITICAL PRESSURE, THE COMBINATION COMPRISING: A WORKING FLUID PREHEATING MEANS; A FLUID SEPARATING AND MIXING VESSEL; A WORKING FLUID RADIANT HEATING MEANS; A WORKING FLUID SUPERHEATING MEANS; MEANS FOR PASSING A THROUGH-FLOW QUANTITY OF THE WORKING FLUID THROUGH SAID PREHEATING MEANS, SAID FLUID SEPARATING AND MIXING VESSEL AND SAID RADIANT HEATING MEANS IN THE ORDER NAMED; MEANS FOR PASSING AN ADDITIONAL QUANTITY OF SAID WORKING FLUID THROUGH SAID SEPARATING AND MIXING VESSEL AND SAID RADIANT HEATING MEANS AND FOR RETURNING SAID ADDITIONAL QUANTITY TO SAID SEPARATING AND MIXING VESSEL; FIRST CONDUIT MEANS FOR PASSING ONLY A QUANTITY OF SAID WORKING FLUID THAT IS EQUAL TO SAID THROUGH FLOW QUANTITY FROM SAID RADIANT HEATING MEANS TO SAID SUPERHEATING MEANS; SECOND CONDUIT MEANS FOR PASSING ONLY A QUANTITY THAT IS EQUAL TO SAID THROUGH FLOW QUANTITY FROM SAID SEPARATING AND MIXING VESSEL TO SAID SUPERHEATING MEANS; VALVE MEANS IN SAID FIRST CONDUIT MEANS AND IN SAID SECOND CONDUIT MEANS; A FIRST POSITION OF SAID VALVE MEANS WHEREBY WHEN OPERATING AT SUPERCRITICAL PRESSURE AND VALVE MEANS ARE OPENED FOR PASSAGE OF SAID THROUGH-FLOW QUANTITY THROUGH SAID FIRST CONDUIT MEANS, AND ARE CLOSED FOR PREVENTING FLOW THROUGH SAID SECOND CONDUIT MEANS AND SAID FLUID SEPARATING AND MIXING VESSEL CAN FUNCTION AS A MIXING DEVICE; AND A SECOND POSITION OF SAID VALVE MEANS, WHEREBY WHEN OPERATING AT SUBCRITICAL PRESSURE SAID VALVE MEANS ARE CLOSED FOR PREVENTING FLOW THROUGH SAID FIRST CONDUIT MEANS, AND ARE OPENED FOR PASSAGE OF SAID THROUGH-FLOW QUANTITY THROUGH SAID SECOND CONDUIT MEANS AND SAID SEPARATING AND MIXING VESSEL FUNCTIONS AS A SEPARATING DEVICE. 